1. Field of the Invention
This invention relates generally to laser systems and more particularly to laser systems which use nonlinear crystals.
2. Description of the Prior Art
Semiconductor diode lasers are of interest for a number of applications such as optical data storage, laser printing and biochemical analysis. One example is the gallium-aluminum-arsenide (GaAlAs) diode laser which generates light in the near infrared range (856 nm in wavelength). In optical data storage systems, the light from the laser diode is focussed onto a spot on the optical disk in order to record each bit of data. The spot size is approximately equal to .lambda./(2*(N.A.)), where .lambda. is the wavelength of the light and N.A. is the numerical aperture of the focusing lens. In typical systems, the N.A. is approximately 0.5 and the resulting spot size is 856 nm in diameter.
It is apparent that if the wavelength of the laser light can be cut in half, the diameter of the spot size will also be cut in half and the overall density of the optical disk may be quadrupled. Unfortunately, laser diodes which produce light in the blue range (430 nm in wavelength) are not available.
One technique to convert light to a higher frequency is known as second harmonic generation (SHG). A laser beam at a first lower frequency is passed through a nonlinear crystal, such as potassium niobate (KNbO.sub.3), which produces a second harmonic (SH) light beam (i.e., a beam at twice the frequency of the original laser beam which entered the nonlinear crystal). This SHG technique is described in articles by M. K. Chun, et al., Applied Physics Letters, Vol. 55, p. 218, July 17, 1989; and P. Gunter, et al., Applied Physics Letters, Vol. 35, p. 461, Sept. 15, 1979. However, since the diode laser's output power is low, techniques to improve the second harmonic generation efficiency are required in order to produce a useful and efficient laser system.
One way to increase the efficiency of the SHG system is to place an optical resonator around the nonlinear crystal. The laser light is reflected back and forth through the crystal inside the resonator in order to generate a substantial amount of the blue light. This technique is described by W. J. Kozlovsky, et al., IEEE Journal of Quantum Electronics, Vol. 24, No. 6, p. 913, June 1988; W. J. Kozlovsky, et al., Optics Letters, Volt. 12, No. 12, p. 1014, December 1987; and A. Ashkin, et al., "Resonant Optical Second Harmonic Generation and Mixing," IEEE Journal of Quantum Electronics, QE-2, p. 109, June 1966; and W. J. Kozlovsky, et al., Applied Physics Letters, Vol. 56, No. 23, p. 2291, June 1990.
In order for efficient SHG to occur, the initial laser beam and the SHG light beam must be phase matched within the nonlinear crystal. This can be achieved by maintaining the crystal temperature at the critical phase matching temperature. U.S. Pat. No. 3,858,056 issued Dec. 31, 1974 by Melamed, et al., shows a temperature control system for a nonlinear crystal. The intensity of the output light is measured by a detector. When the output light intensity is at a maximum the crystal is at the correct temperature. A problem with this type of system is that the light intensity error signal does not indicate in which direction to adjust the crystal temperature. The system must sample at least two temperature points in order to determine the correct direction to adjust the temperature. This type of system is very unstable and it is very difficult to maintain the exact optimum temperature. This is a critical problem where the temperature must be maintained within a small range of fractions of degrees C.
What is needed is a simple temperature control system which will maintain the nonlinear crystal at the exact optimum temperature.